1. Field of the Invention
The present invention relates to an image pickup device.
Priority is claimed on Japanese Patent Application No. 2011-018200, filed Jan. 31, 2011, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
As an image pickup device using an analog-to-digital (AD) conversion method related to an example of the related art, a configuration disclosed in Japanese Unexamined Patent Application, First Publication No. 2006-340044 (hereinafter referred to as Patent Document 1) is known. First, a configuration and operation of the image pickup device disclosed in Patent Document 1 will be described.
FIG. 18 is a block diagram illustrating a schematic configuration of a (complementary) metal oxide semiconductor ((C)MOS) image pickup device using an AD conversion method disclosed in Patent Document 1. An image pickup device 1001 includes an image pickup unit 1002, a vertical selection unit 1012, a read current source unit 1005, an analog unit 1006, a column processing unit 1015, a reference signal generation unit 1016, a horizontal selection unit 1014, an output unit 1017, a change unit 1018, and a timing control unit 1020.
The timing control unit 1020 controls parts such as the vertical selection unit 1012, the read current source unit 1005, the analog unit 1006, the column processing unit 1015, the reference signal generation unit 1016, the horizontal selection unit 1014, and the output unit 1017. The image pickup unit 1002 includes unit pixels 1003 having photoelectric conversion elements arranged in a matrix, generates a pixel signal corresponding to the amount of an incident electromagnetic wave, and outputs the pixel signal to a vertical signal line 1013 provided in each column.
Upon driving of each unit pixel 1003 of the image pickup unit 1002, the vertical selection unit 1012 controls a row address or a row scan of the image pickup unit 1002 via a row control line 1011. The horizontal selection unit 1014 controls a column address or a column scan of a column AD conversion unit 1030 of the column processing unit 1015. The read current source unit 1005 is a current source for reading the pixel signal from the image pickup unit 1002 as a voltage signal. The analog unit 1006 executes amplification or the like, if necessary.
The column processing unit 1015 has the change unit 1018 and the column AD conversion unit 1030 provided in each column of the image pickup unit 1002. The change unit 1018 is turned on (activated), and hence is short-circuited to a voltage source, which supplies a predetermined voltage. The column AD conversion unit 1030 converts an analog signal, which is a pixel signal output from each unit pixel 1003 of the image pickup unit 1002 in each column, into digital data, thereby outputting the digital data. The reference signal generation unit 1016 includes, for example, an integral circuit or a digital-to-analog conversion (DAC) circuit, and generates a reference signal Ramp, which varies in an inclined shape according to the passage of time.
Next, a configuration of the column AD conversion unit 1030 will be described. FIG. 19 is a block diagram illustrating a configuration of the column processing unit 1015 including the column AD conversion units 1030. The column AD conversion units 1030 all have the same configuration, and each column AD conversion unit 1030 is configured to have a comparison unit 1031 and a measurement unit 1032.
The comparison unit 1031 is a comparator circuit having a differential amplifier which is generally well-known as a basic configuration. The comparison unit 1031 compares a pixel signal output from the unit pixel 1003 of the image pickup unit 1002 to the reference signal Ramp, outputs a high (H) level, for example, when the reference signal Ramp is larger than the pixel signal, and outputs a low (L) level, for example, when the reference signal Ramp is smaller than the pixel signal.
The measurement unit 1032 includes an up/down-counter circuit, and measures a comparison time of the comparison unit 1031 from a comparison start to a comparison end. Thereby, a measurement value of the comparison time corresponding to the amount of a pixel signal can be obtained. The horizontal selection unit 1014 includes a shift register, a decoder, or the like, and controls a column address or a column scan of each column AD conversion unit 1030 in the column processing unit 1015. Thereby, AD-converted digital data is output to the output unit 1017 by way of a horizontal signal line.
A comparison operation by the comparison unit 1031 is initiated after voltages of two input terminals of the differential amplifier constituting the comparison unit 1031 are reset (balanced). The change unit 1018 is provided in the image pickup device 1001 to prevent a defect in which an output of the comparison unit 1031 is not inverted, or the output of the comparison unit 1031 is inverted immediately after an input of the reference signal Ramp, by slight variation remaining in the voltages of the two input terminals of the differential amplifier constituting the comparison unit 1031 after the reset operation.
Next, an AD conversion operation will be described. Description of a specific operation of the unit pixel 1003 is omitted, but a reset level and a signal level are output as pixel signals from the unit pixel 1003.
First, the voltages of the two input terminals of the differential amplifier constituting the comparison unit 1031 are reset (balanced) once reading of the reset level from the unit pixel 1003 is stable. Subsequently, the change unit 1018 applies a predetermined voltage to an input terminal to which the reference signal Ramp has been given. Thereafter, the comparison unit 1031 compares the reference signal Ramp to the pixel signal using the predetermined voltage as a voltage of a comparison start. The measurement unit 1032 performs measurement in a count-down mode, and a measurement value of a comparison end time becomes digital data at the reset level.
Subsequently, when the signal level from the unit pixel 1003 is read, the reset operation in the comparison unit 1031 and the change operation by the change unit 1018 are not performed. Once the reading of the signal level from the unit pixel 1003 is stable, the comparison unit 1031 compares the reference signal Ramp to the pixel signal using the predetermined voltage as the voltage of the comparison start. The measurement unit 1032 performs measurement in a count-up mode, and a measurement value of the measurement unit 1032 of the comparison end time becomes a digital data of a signal component (a signal obtained by subtracting the reset level from the signal level).
As described above, it is possible to AD-convert the pixel signal. In addition, even when slight variation remains in the voltages of the two input terminals of the differential amplifier constituting the comparison unit 1031 after the reset operation, the change unit 1018 applies the predetermined voltage to the input terminal to which the reference signal Ramp has been given, so that the output of the comparison unit 1031 can be reliably inverted during the comparison operation because the voltage of the input terminal to which the reference signal Ramp has been given is higher than the voltage of the input terminal to which the pixel signal has been given.
In the image pickup device disclosed in Patent Document 1, the voltage of the input terminal to which the reference signal Ramp has been given is changed, but an operation of changing the voltage of the input terminal to which the pixel signal has been given is not disclosed.
(1) Configuration of Change Unit
It is preferable to use an n-channel MOS (NMOS) transistor (hereinafter referred to as NMOS) for the input terminal of the differential amplifier constituting the comparison unit for the speed-up of the comparison unit and use NMOS as a switch element constituting the change unit for the speed-up of the change unit. When the comparison unit and the change unit using these configurations are applied to a general image pickup device, it is necessary to increase and change a voltage of an input terminal by providing a predetermined voltage to the input terminal to which the reference signal is given between the two input terminals of the differential amplifier after the reset operation so as to reliably perform the comparison operation. However, the voltage of the input terminal may not be changed because NMOS constituting the change unit is not turned on (activated) by the predetermined voltage given to the input terminal, or the change may be time-consuming because a resistance component is large even when NMOS is turned on. Although the change unit may also be constituted by NMOS having a small threshold, an additional step of a semiconductor process that is cost-ineffective is necessary.
(2) Configuration of Directly Changing Voltages of Input Terminals of Differential Amplifier
In order to solve the problem occurring in (1), the change unit may be constituted by a p-channel MOS (PMOS) transistor (hereinafter referred to as PMOS). It is possible to sacrifice small size to implement the speed-up of the change unit by increasing a size of PMOS. However, when the voltages of the input terminals of the differential amplifier are directly changed, the following problem occurs. The voltages of the two input terminals of the differential amplifier after the reset are not completely the same as a reset voltage VRST, and include reset variation ΔVRST due to a threshold of the transistor or the like. In general, variations of manufacturing conditions between adjacent columns of the image pickup device are substantially the same. However, the variations of the manufacturing conditions may be increased between all columns of the image pickup device and the reset variation ΔVRST may be about 100 mV. That is, when the voltages of the input terminals of the differential amplifier are directly changed differently from when the voltages of the input terminals of the differential amplifier is changed via a capacitive element or the like, it is necessary to make a change to a voltage including the reset variation ΔVRST. Because an additional comparison time for the reset variation ΔVRST between all the columns of the image pickup device is necessary, the comparison time is lengthened and the speed-up of AD conversion is difficult.
(3) Configuration of Reference Signal Generation Unit
The voltage of the input terminal to which the reference signal is given may be changed by controlling the reference signal. In this case, although the problems occurring in (1) and (2) can be suppressed, a voltage change function needs to be embedded in the reference signal generation unit and the configuration or its control becomes complex.